Backend Wishlist Implementation

[Corangit]

Created wishlist.json - Hashmap/Dict where User ID key contains list of products on wishlist
Need service and business layer to have users be able to add products to their wishlists.

This PR adds a complete backend wishlist system, including repository logic, API endpoints, data validation, and full test coverage.

Key Additions
WishlistRepository
Stores wishlists as a dictionary: user_id → [product_ids]
Supports loading/saving data, retrieving wishlists, and adding items without duplicates.

API Endpoints
GET /wishlist/{user_id} — returns a user’s wishlist
POST /wishlist/add — adds a product to a user’s wishlist
Includes a new WishlistItem Pydantic model for validation.

Testing
The test_wishlist.py suite was rewritten to match the new design and now tests:
Loading/saving repo data
Handling of missing users/files
Adding to wishlist
GET/POST endpoint behavior using dependency overrides
All tests pass.

Other Improvements
Fixed missing imports and routing issues
Cleaned up dependency injection
Ensured consistent, predictable JSON data structure

[Corangit]

Waiting for a review but everything should work fine, I can complete conflicts no problem but if someone else wants to:
#for the conflict, line 6-12 keep incoming change but add wishlist_router to backend.routers import. Second conflict lines 25-30 keep both but take out commented export router and keep uncommented wishlist and export router includes

Otherwise I will resolve conflicts after review

[frasermuller]

Just fixed merge conflicts, was super quick just needing to change main.py to include all the routers